1. Field of the Invention
This invention relates to solid state, low-level light detectors in general and specifically to room temperature, low-level visible and shorter wavelength imagers using III-V compound semiconductor photodetectors.
2. Description of the Related Art
Visible-light sensors transform incident light in the visible region of the spectrum (wavelengths from approximately 0.38 to 0.78 microns) into electrical signals that are used for data collection, processing, and storage. For example, the such sensors can capture images for video or still-frame imaging. Conventional solid-state sensors usually employ silicon photodetectors because they are inexpensive, exhibit adequate signal bandwidth, are sensitive to visible wavelengths, and exhibit a high degree of pixel-to-pixel uniformity when used in an imaging array.
Silicon photodetectors exhibit significant deficiencies when applied at room temperature for visible wavelength detection, however. At or near room temperature, these detectors have a characteristic xe2x80x9cdark currentxe2x80x9d which is relatively high and causes prominent noise characteristics. The dark current is a fundamental consequence of the detector physics: common silicon photodetectors exhibit an energy bandgap in the 1.1 eV range, which is well below the threshold for visible wavelength absorption. This low bandgap gives rise to a large dark current (at normal room temperatures). Silicon photodetectors which are compatible with. low to moderate cost production also have no gain: i.e. each incident photon generates at most a single electron. The combination of low gain and large dark currents limits the practical application of Silicon photodiode detectors to moderate to bright light conditions, unless active cooling is used. At low light levels and at room temperature such detectors generate inadequate signal-to-noise ratios (SNR).
In low-light-level applications, the standard silicon photodiode detector is often replaced with a silicon avalanche detector to facilitate gain within the detector so that conventional detector interface amplifiers and ancillary interface circuits can be used to read out the data at video frame rates with a high SNR. Such applications of avalanche photodiodes are disclosed by U.S. Pat. No. 5,146,296 to Huth and U.S. Pat. No. 5,818,052 to Elabd, for example. Unfortunately, the fabrication of avalanche photodiodes is much more difficult and expensive than standard photodiodes, and supplemental amplification is also often required. Currently available avalanche photodiodes exhibit relatively poor uniformity and have limited sensitivity due to their low quantum efficiency. They are also inherently non-linear in their response to light, which is undesirable in many applications.
Alternative imaging systems are known which use an array of avalanche detectors, various phosphors or intensifiers such as microchannel plates to amplify the available electrons for subsequent detection in enhanced charge coupled devices (CCDs); but these intensified devices respond inadequately to broadband radiation otherwise available to maximize sensitivity. All such CCDs and other metal-insulator-semiconductor (MIS) devices have surface states at the semiconductor/insulator interface that cause spontaneous generation of dark current. Furthermore, intensified CCDs are damaged by the soft x-rays associated with electron bombardment. This damage is manifest as even higher dark current. This current provides an undesirable background signal that reduces dynamic range, both by consuming charge handling capacity and by adding noise. CCD Manufacturers employ various schemes to suppress dark current, such as that described by Saks, xe2x80x9cA Technique for suppressing Dark Current Generated by Interface States in Buried Channel CCD Imagers,xe2x80x9d IEEE Electron Device Letters, Vol. EDL-1, No. 7, July 1980, pp. 131-133. Nevertheless, mid-gap states are always present that result in unacceptable dark current for room temperature operation of silicon-based low-light-level image sensors.
A further problem with prior detectors arises from their spectral response characteristics. A large mismatch between the photoresponse required to facilitate CIE (Commission International de LeClairage) chromaticity and the actual spectral response of silicon photodetectors is illustrated in FIG. 1. Enabling color reconstruction to minimum CIE specifications for best color fidelity requires high, broadband response from about 380 nm to 780 nm. The photoresponse for scientific-grade silicon detectors instead extends to the bandgap of silicon, at beyond 1 xcexcm. As FIG. 1 shows, maximizing the photo-response in the red part of the spectrum typically tends to compromise blue response and vice versa. For example, compare the response curve 16 of the xe2x80x9cMIT EPIxe2x80x9d device, as compared to response curve 18 of the xe2x80x9cMIT High-rhoxe2x80x9d (i.e., high-resistivity) device, in FIG. 1. (Both devices are available from Lincoln Laboratory of Massachusetts Institute of Technology, in Lexington, Mass.) The high-rho device shows markedly better red response, while the EPI device is more sensitive in the blue region, but neither is inherently color balanced.
In addition to chromaticity problems with conventional detectors, the unneeded photoresponse in the near infrared from 780 nm to beyond 1 xcexcm creates the requirement for an xe2x80x9cinfrared-blocking filterxe2x80x9d in such cameras to suppress the photoresponse beyond the CIE requirement, thereby increasing both camera cost and complexity. Finally, absorption of photons in silicon via its indirect bandgap mandates thick detectors and/or significant modifications to CCD and CMOS processes for monolithic imagers to fully absorb the longer wavelength photons. This thickness requirement effectively increases the depletion volume subject to dark current generation, thereby further increasing the minimum dark current. The decreasing absorption at longer wavelengths also increases the signal processing required for maximizing color fidelity because of the resulting crosstalk from pixel to pixel and color level imbalance.
In view of the above problems, the present invention is a high-sensitivity photodetector for detecting radiation in the visible or shorter wavelength regions of the spectrum, suitable for operation at room-temperature. The photodetector includes (a) a compound semiconductor photodiode which generates a detector current in response to incident photons, the photodiode biased below its avalanche breakdown threshold, comprising III-V elemental components and having a bandgap with transition energy higher than the energy of infrared photons; and (b) a detector interface circuit at each pixel, arranged to receive a signal from the photodiode junction and to amplify said signal.
Preferably, the photodetector has its photodiode junction integrated in a first microstructure on a first substrate, and its interface circuit in a second microstructure on a second substrate. The first and second microstructures are then joined in a laminar, sandwich-like structure. The first and second microstructures communicate via electrically conducting contacts.
In one embodiment, the photodetectors are integrated in an imaging array for use at room temperatures to detect images from low-level visible, ultraviolet or shorter illumination, and suitable for use at video frame rates. Such imaging array is made up of a plurality of addressable photodetecting cells. Each cell includes a compound semiconductor photodiode which linearly generates a detector current in response to incident photons, the photodiode biased below its avalanche breakdown threshold, comprising III-V elemental components and having a bandgap with transition energy higher than the energy of infrared photons. Each cell also includes a high trans-impedance interface circuit at each pixel, arranged to receive a signal from the photodiode junction and amplify the signal.
In some embodiments, the interface circuit of at least some cells have independently variable gain. Thus, the gain at each pixel can be set to compensate for non-uniform photodiode response across the array.
The photodetector and array of the invention provide single-photon sensitivities, with higher signal-to-noise ratios at video frame rates and room temperature than previously possible, for detecting and imaging radiation in the visible region of the spectrum.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: